Role of micropipes in the formation of pores at foreign polytype boundaries in SiC crystals

The role of micropipes in pore formation in SiC crystals with foreign polytype inclusions is studied by means of synchrotron phase sensitive radiography, optical and scanning electron microscopies, and color photoluminescence. The pores at the inclusion boundaries are revealed, and their shapes and locations are analyzed. It is found that the pores arise due to the attraction of micropipes by the foreign polytype interfaces, followed by micropipe coalescence. The observed pores have tubular or slit shapes. Tubular pores nucleate at the inclusion corners, where the inclusion-associated stresses are concentrated. Slit pores spread between them and follow the shape of the inclusion boundaries. We explain the observations within a two-dimensional model of elastic interaction between micropipes and inclusion boundaries, which accounts for free surfaces of micropipes.open119sciescopu

Features in Phase-Contrast Images of Micropipes in SiC in White Synchrotron Radiation Beam

An interesting feature in phase-contrast images of micropipes in silicon carbide in white synchrotron radiation beam was experimentally studied and theoretically explained. This feature consists in that a change in micropipe cross-section sizes does not lead to changes in its image sizes, but has an effect only on the contrast. The experiment was performed on the synchrotron radiation source in Pohang, South Korea. On the one hand, this effect is explained by a small phase progression caused by the micropipe, and, on the other hand, by satisfying the conditions for Fraunhofer diffraction, when the transverse micropipe size is smaller than the first Fresnel zone diameter. As a rule, the near-field conditions are satisfied in X-ray optics when only object edges are imaged. However, micropipes are so small that the standard edge theory is inapplicable. A universal intensity distribution profile was obtained for micropipes with very small cross sections.X1133sciescopu

Computer Simulation of Phase-Contrast Images in White Synchrotron Radiation Using Micropipes in Silicon Carbide

A method of computer simulation of phase-contrast images in white synchrotron radiation has been developed to determine the section parameters of micropipes in silicon carbide. The experiments have been carried out using the third-generation synchrotron radiation source the Pohang Light Source (South Korea). The effective spectrum of the synchrotron radiation that forms of an image has been shown to have a relatively sharp maximum at an energy of 16 keV, which makes it possible to conserve coherency within the required limits. A computer program has been developed that automatically determines the diameters of an elliptic section of a micropipe from the condition of coincidence of calculated and experimental profiles. It has been shown that the studied micropipes have a strongly stretched elliptic section that can twist when moving along the pipe axis.X1176sciescopu

Capsule-like voids in SiC single crystal: Phase contrast imaging and computer simulations

The results of observation of capsule-like voids in silicon carbide (6H-SiC) single crystal by means of a phase contrast imaging technique with synchrotron radiation at the Pohang Light Source as well as computer simulations of such images are presented. A pink beam and a monochromated beam were used. The latter gives more pronounced images but they still are smoothed due to a finite detector resolution and the spatial coherence of the beam. Sizes and a structure of far field images are different from these of the objects. The computer simulations allow us to reproduce a shape and a size of the capsule-like void. (C) 2014 Author(s).open1111sciescopu

Study of micropipe structure in SiC by x-ray phase contrast imaging

Phase contrast images of dislocation micropipe in SiC crystal are experimentally studied at various distances from the sample using synchrotron white beam. Computer simulation of these images enabled us to understand the peculiarities of image formation and measure the diameter of the micropipe. The phase contrast imaging of micropipes without monochromator is explained by the absorption of x rays in a thick (490 mu m) SiC crystal, effectively forming a high brilliance radiation spectrum with a pronounced maximum at 16 keV. (C) 2007 American Institute of Physics.open112220sciescopu

Far-field x-ray phase contrast imaging has no detailed information on the object

We show that x-ray phase contrast images of some objects with a small cross-section diameter d satisfy a condition for a far-field approximation d << r(1) where r(1) = (lambda z)(1/2), lambda is the x-ray wavelength, z is the distance from the object to the detector. In this case the size of the image does not match the size of the object contrary to the edge detection technique. Moreover, the structure of the central fringes of the image is universal, i.e. it is independent of the object cross-section structure. Therefore, these images have no detailed information on the object.X1177sciescopu

On the cause of a contrast change in the SR images of micropipes in SiC

We propose a theoretical explanation for the experimentally observed feature in phase contrast images of micropipes in silicon carbide in a white synchrotron radiation (SR) beam, which consists in the fact that the contrast of various micropipes or various regions of the same micropipe is of different colors. The contrast is most often white (the intensity is higher than the background) at the center and black at the edges; however, the sign sometimes changes, and the contrast becomes black at the center and white at the edges. We discuss the results of experiments performed at the SR source in Pohang, Republic of Korea. The cause of the contrast change can be a change in the angle between the micropipe axis and the SR beam direction. At not overly small angles, the phase progression in a section of the micropipe is small and the contrast is standard. If the angle becomes very small, the size of the longitudinal section of the micropipe by the beam increases, which leads to wave field oscillations in the region of the section. At large distances in the white beam, these oscillations are averaged, and averaging results in black contrast.X1144sciescopu

Analytic determination of the three-dimensional distribution of dislocations using synchrotron X-ray topography

A technique, using a symmetric reflection via azimuthal rotation of a sample, is presented for characterization of the three-dimensional distribution of dislocations in single crystals. An analytic formula is derived to transform the three-dimensional geometry of a straight dislocation into its two-dimensional projection onto the detector plane. By fitting topographs to the formula, the orientations and locations of dislocations are quantitatively determined. The dislocations in a thermally stressed Si wafer are examined as an example.open113sciescopu

Study of a Macrodefect in a Silicon Carbid Single Crystal by Means of X-Ray Phase Contrast

The morphology of a macrodefect in a single-crystal silicon carbide wafer has been investigated by the computer simulation of an experimental X-ray phase-contrast image. A micropipe, i.e., a long cavity with a small (elliptical in the general case) cross section, in a single crystal has been considered as a macrodefect. A far-field image of micropipe has been measured with the aid of synchrotron radiation without a monochromator. The parameters of micropipe elliptical cross section are determined based on one projection in two directions: parallel and perpendicular to the X-ray beam propagation direction, when scanning along the pipe axis. The results demonstrate the efficiency of the phase contrast method supplemented with computer simulation for studying such macrodefects when the defect position in the sample volume is unknown before-hand.11sciescopu

Si1-xGex single crystals grown by the Czochralski method: Defects and electrical properties

Defects in Si1-xGex single crystals (2-8.5 at.% Ge) grown by the Czochralski method are investigated by synchrotron white beam topography and phase contrast imaging techniques. As the Ge concentration increases, dislocation structure evolves from individual dislocations to slip bands and sub-grain boundaries. We discuss the effect of dislocations on the electrical characteristics such as resistivity rho(nu), the Hall hole mobility mu(p) and carrier lifetime tau(e). Diodes are fabricated by bonding p-Si1-xGex to n-Si wafers to investigate I-V characteristics and reverse recovery process. I-V characteristics are not deteriorated in spite of a five times decrease in tau(e) with Ge concentration. A small reverse recovery time (determined by the accumulated charge) can be achieved for an optimised preset Ge concentration.ungraded110sciescopu